This invention relates primarily to producing natural gas from a well formed in an earth formation, and more particularly to a new and improved gas recovery system, method and gas recovery cycle during which an evacuation pressure is applied to three chambers within the well and a hydrocarbon-bearing zone of the earth formation to assist natural formation pressure in producing natural gas and liquid into the well. The resulting three chamber evacuation phase augments the effect of natural earth formation pressure to produce gas and liquid at a higher volumetric rate, thereby increasing the efficiency of gas production, lifting the liquid from the well by more efficient and shorter recovery cycles, and improving efficiency by better use and conservation of the existing pressure states within the chambers during the recovery cycle, among other things.
The production of oil and natural gas depends on natural pressure within the earth formation at the bottom of a well bore, as well as the mechanical efficiency of the equipment and its configuration within the well bore to move the hydrocarbons from the earth formation to the surface. The natural formation pressure forces the oil and gas into the well bore. In the early stages of a producing well when there is considerable formation pressure, the formation pressure may force the oil and gas entirely to the earth surface without assistance. In later stages of a well""s life after the formation pressure has diminished, the formation pressure is effective only to move liquid and gas from the earth formation into the well. The formation pressure pushes liquid and gas into the well until a hydrostatic head created by a column of accumulated liquid counterbalances the natural earth formation pressure. Then, a pressure equilibrium condition exists and no more oil or gas or water flows from the earth formation into the well. The hydrostatic head pressure from the accumulated liquid column chokes off the further flow of liquid into the well bore, causing the well to xe2x80x9cdie,xe2x80x9d unless the accumulated liquid is pumped or lifted out of the well.
By continually removing the liquid, the hydrostatic head pressure from the accumulated column of liquid remains less than the natural earth formation pressure. Under such circumstances, the natural earth formation pressure continues to move the liquid and gas into the well, allowing the liquid and gas to be recovered or produced. At some point when the natural earth formation pressure has diminished significantly, the cost of removing the liquid diminishes the value of the recovered oil and gas to the point where it becomes uneconomic to continue to work the well. Under those circumstances, the well is abandoned because it is no longer economically productive. A deeper well will require more energy to pump the liquid from the well bottom, because more energy is required to lift the liquid the greater distance to the earth surface. Deeper wells are therefore abandoned with higher remaining formation pressure than shallower wells.
To keep wells in production, it is necessary to remove the accumulated liquid to prevent the liquid from choking off the flow of gas into a gas producing well, but because a considerably greater volume of gas is usually produced into a well compared to the amount of liquid produced into the well, the greater volume of gas can be recovered more economically by removing a relatively lesser volume of the accumulated liquid. Consequently, there may be an economic advantage to recovering natural gas at the end of a well""s lifetime, because the gas is more economically recovered as a result of removing a relatively smaller amount of accumulated liquid. These factors are particularly applicable to recovering gas from relatively deep wells.
Gas pressure lift systems have been developed to lift liquid from wells under circumstances where mechanical pumps would not be effective or not sufficiently economical. In general, gas pressure lift systems inject pressurized gas into the well to force the liquid up from the well bottom, rather than rely on mechanical pumping devices to lift the liquid. The injected gas may froth the liquid by mixing the heavier density liquid with the lighter density gas to reduce the overall density of the lifted material. Alternatively, xe2x80x9cslugsxe2x80x9d or shortened column lengths of liquid are separated by bubble-like spaces of pressurized gas, again reducing the overall density of the lifted material. In both cases, the amount of energy required to lift the material is reduced, or for a given amount of energy it is possible to lift material from a greater depth.
One problem with injecting pressurized gas into a well casing is that the pressurized gas tends to oppose the natural formation pressure. The injected gas pressure counterbalances the formation pressure to inhibit or diminish the flow of liquids and natural gas into the well. Once the gas pressure is removed, the natural earth formation will again become effective to move the liquid and gas into the well. However, because the casing annulus is pressurized for a significant amount of time during each production cycle, the net effect is that the injected gas pressure diminishes the production of the well. Stated alternatively, producing a given amount of liquid and gas from the well requires a longer time period to accomplish. Such reductions in the production efficiency in the later stages of the well""s life may be so significant that it becomes uneconomical to work the well, even though some amount of hydrocarbons remain in the formation.
One particularly advantageous type of pressurized gas lift apparatus is described in U.S. Pat. 5,911,278, by the inventor hereof. The gas lift apparatus described in U.S. Pat. 5,911,278 is primarily intended for lifting oil from a well, rather than natural gas, but it is also effective for producing natural gas. The gas lift apparatus described in this patent uses a production tube inserted into the well casing with a lift to be located within the production tube. A one-way valve located at the bottom of the production tube responds to pressure differentials to selectively isolate the earth formation from the pressure of gas injected in the production tube. By confining the injected pressurized gas within the production tubing, and by not applying the injected pressurized gas directly to the earth formation, the natural earth formation pressure is not impeded to restrict or prevent the flow of the liquid and gas into the well during a significant portion of the recovery cycle. Instead, the earth formation pressure, diminished as it may be at the later stages of a well""s life, remains available to move the liquid and gas into the well for a significant portion of the recovery cycle.
Another improvement available from U.S. Pat. No. 5,911,278 is that an evacuation pressure is applied to the casing annulus and the hydrocarbon-bearing zone of the earth formation during certain phases of the recovery cycle. The diminished or evacuation pressure has the effect of augmenting the natural earth formation pressure, thereby enhancing the flow of liquids and gas into the well. As a result, the production efficiency of the well is enhanced, which is particularly important in the later stages of a well""s life where the natural earth formation pressure has already diminished.
This invention is directed to an improved recovery cycle for a pressurized gas lift apparatus, such as the type described in U.S. Pat. No. 5,911,278. In the present invention, an additional phase is included within the recovery cycle. The additional phase involves the evacuation of all three chambers created by the well casing, a production tubing within the well casing, and a lift tubing within the production tubing. The evacuation of all three chambers during the three chamber evacuation phase of the recovery cycle has the benefit of enhancing natural gas production by augmenting earth formation pressure to recover the gas at a higher rate within a given period of time. In addition, the three chamber evacuation phase facilitates a condition where the produced natural gas may be delivered to a sales line or pipeline that has a relatively high pressure.
The present invention involves a method of recovering natural gas from a well by executing a multiple-phase gas recovery cycle. The gas recovery cycle includes a liquid capture phase in which pressurized gas moves liquid from the well into a production chamber defined within a production tubing inserted into the well, a liquid removal phase in which pressurized gas lifts liquid out of the well through a lift chamber defined by a lift tubing inserted at least partially within the production chamber, and a production phase during which natural gas is removed from the well in a casing chamber defined by production tubing and a casing within the well. During the production phase the gas is pressurized and flowed through the production chamber and the lift chamber for delivery to a sales conduit. In addition, the gas recovery method and cycle includes a new and improved three chamber evacuation phase which is executed by applying relatively low pressure within the casing chamber, production chamber and lift chamber after completion of the liquid removal and production phases and before execution of the liquid capture phase. The relatively low pressure applied within all three chambers augments the natural earth formation pressure to produce natural gas and liquid into the well at a greater rate than would otherwise result. The four phases of the gas recovery cycle are arranged to take advantage of the greater production rate by more rapidly removing the liquid from the well bottom to maintain natural gas production and increase the volumetric rate of its production. Moreover, the three chamber evacuation phase permits the produced natural gas to be pressurized, if necessary, to be delivered directly into a relatively high-pressure sales conduit or pipeline.
Other beneficial aspects of the three chamber evacuation phase in the gas recovery cycle include flowing at least some of the natural gas from the casing chamber directly to the sales conduit, and moving accumulated liquid from the casing chamber into the production chamber during the three chamber evacuation phase and prior to executing the liquid capture phase. The three chamber evacuation phase may be selectively terminated upon sensing a predetermined amount of natural gas flow from the casing chamber and a predetermined pressure of natural gas in the casing chamber, under conditions which correlate to an amount of accumulated liquid which may be lifted from the well bottom without exceeding the capacity of a compressor used to lift the accumulated liquid.
Another aspect of the present invention involves a gas recovery method that includes a well evacuation phase in a gas recovery cycle during which relatively low gas pressure is applied throughout the well and on an earth formation from which the gas and liquid produced at a bottom of the well, thereby augmenting the natural earth formation pressure to increase the volumetric flow rate of the natural gas and liquid into the well. The gas recovery cycle beneficially maintains the increased volumetric flow by increasing the volumetric removal rate of the liquid from within the well. Moreover, the well evacuation phase facilitates pressurizing of the gas produced from the well for delivery to a high-pressure sales conduit, if necessary.
Another aspect of the present invention involves a system controller in a gas recovery apparatus which has been programmed to control a compressor and the gas flow path established through controllable valves for the purpose of executing a gas recovery cycle involving an improved three chamber phase or a well evacuation phase of the nature described.
A more complete appreciation of the present invention and its scope may be obtained from the accompanying drawings, which are briefly summarized below, from the following detail descriptions of presently preferred embodiments of the invention, and from the appended claims.